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Science: Lizards navigate off the top of their heads

By CLARE PUTNAM

Lizards find their way home by keeping two eyes on what lies ahead and
one on the sky, it seems. If you remove a lizard from its home, it will
find its way back there fairly quickly. The distances covered – between
180 and 280 metres in species investigated so far – are modest compared
with those achieved by their descendants, the birds, but they do suggest
that lizards must have a ‘sense of direction’.

Barbara Ellis-Quinn and Carol Simon, two biologists from the City University
of New York, have now established that this sense is based in the animals’
third or ‘parietal’ eye. The eye is on top of the animals’ heads, and is
a functional light receptor which contains a cornea, lens and retina (Behavioural
Ecology and Sociobiology, vol 28, p 397).

The researchers studied Yarrow’s Spiny Lizard (Sceloporus jarrovi)
in Arizona. They covered the parietal eyes of 40 lizards with a layer of
paint to see whether this would affect their homing sense. Then they put
the lizards in a bag and released them 150 metres away. Painting the eye
reduced the number that successfully returned to their home area from 61
to 21 per cent, while putting paint elsewhere on the lizards’ heads had
no effect.

To follow the lizards’ movements in more detail, the researchers then
put small radio transmitters in some lizards’ abdominal cavities. Tracking
them with a receiver after releasing them 150 metres from home confirmed
the importance of the parietal eye. When this was covered by paint the lizards
moved around at random, whereas unpainted controls began to move homeward
within half an hour of being released.

In a final experiment, Ellis-Quinn and Simon tried to find out how the
lizards used their parietal eye. To do this they kept one group of lizards
on a normal light-dark cycle and another group with their light-dark cycle
six hours ahead. When these animals were released 150 metres from home the
controls went straight back, but those kept six hours ahead veered anti-clockwise,
and eventually went 90 degrees off course. The researchers conclude that
these lizards use the position of the Sun to direct them homeward. They
also suggest that the parietal eye helps by detecting the degree of polarisation
of sunlight. Honey bees use a similar mechanism to navigate.